Analytical investigation of the composition of plasma-induced functional groups on carbon nanotube sheets

To increase the applicability of carbon nanotubes (CNTs) oxygen-containing functional groups were generated on their widely inert surface by using glow-discharge plasmas. CNT-sheets (bucky papers) produced from the powder-like raw material were used as substrates allowing for a more defined characterization of one and the same surface by different analytical techniques. The plasma composition was analyzed by optical emission spectroscopy. Since the actual composition of the plasma-induced functional groups has still not been completely resolved, we performed an in-depth characterization of the treated samples by X-ray photoelectron (XPS) and Raman spectroscopy as well as electron spin resonance measurements. To overcome limitations of the XPS-analysis in distinguishing between groups featuring nearby binding energies, alcohol-, keto-/aldehyde-, and carboxyl-groups were tagged by derivatization techniques using fluorine-containing reagents (trifluoroaceticanhydride, trifluoromethylphenylhydrazine, and trifluoroethanol). Differential spectra were calculated to enhance the accuracy of the deconvolution of the XPS-spectra. This enabled us to determine dependencies of the plasma parameters, i.e. treatment time, process pressure, and gas composition (mixtures of Ar, O₂, H₂O, and H₂), on the composition of the generated functional groups as well as an up to 6-fold enhancement in derivatizable groups for switching process gas from Ar/O₂ to Ar/H₂O.